Metal strip may be heat treated as an endless belt passing horizontally or vertically (looping tower) through a furnace after which the strip is rewound as a coil. Alternatively the strip may be heat treated in a batch furnace with the strip tightly wound as coils vertically stacked one on top of the other.
Batch coil annealing furnaces (sometimes called "box annealing furnaces" or "bell shaped" furnaces) have been long used and are well known in the industry. Basically such furnaces comprise a base upon which steel coils are stacked vertically, edge upon edge, and over which a removable inner cover is placed. (The invention is applicable to all metals. However, the discussion will be limited to annealing, cold rolled, sheet steel typically used in the automotive and appliance industries.) An outer cover in turn is placed over the inner cover. The covers are removably sealed to the base. The outer cover contains some form of heat mechanism, typically gas fired burners, which heat the inner cover, and the inner cover in turn radiates the heat to the work. Batch coil annealing processes in the steel mill industry typically take anywhere from about 20 hours to as long as several (3) days to complete.
Traditionally, continuous strip annealing process has been viewed as superior to the batch annealing process on a quality or quality controlled basis. Substantively, it is believed that the work quality gap between the two processes was narrowed significantly with the introduction of pure hydrogen into the inner cover during the heating stage for a variety of chemical and metallurgical reasons which will not be set forth in detail nor commented on further herein since the present invention does not claim to have invented the use of hydrogen in the batch coil annealing process. At the same time however, the use of hydrogen in the steel mill environment raises serious safety concerns since any mixing of hydrogen with oxygen at standard atmospheric pressure will produce a highly combustible gas mixture.
Because of safety concerns, after the coils are placed on the base and covered with the inner cover and while the work is being heated to its annealing transformation temperature, the air has to be purged from the annealing cover. If the annealing system is using some hydrogen-nitrogen atmosphere gas, i.e., an HNX.TM. gas, a number of volume changes of atmosphere must be employed before heating at elevated temperatures can occur. If a pure hydrogen atmosphere is to be used purging will take even longer and be more expensive. When pure hydrogen is used, the base must first be completely purged with an inert gas such as nitrogen before a switch to hydrogen is made. This is why in some systems which use hydrogen-nitrogen atmospheres, the hydrogen to nitrogen ratio becomes increasingly shifted to hydrogen as the process continues. Further when pure hydrogen systems are used, once the heating is accomplished, full cooling with air at ambient temperature can not take place until the hydrogen has been purged or diluted with an inert gas such as nitrogen. In general summary, the use of hydrogen enhanced the product quality of strip which has been annealed in a coiled state, but the cycle time was lengthened and the process cost increased because of the additional cost required to supply the inert, purged gas. Though nitrogen is a relatively inexpensive gas, the gas volume used within the cover and the number of gas changeovers required does constitute an economic process concern or consideration.
Apart from process considerations relating to gas expense and process time, there is the overriding safety concern to produce an effective furnace seal given the explosion potential resulting from the use of hydrogen, especially pure hydrogen, as a convective, furnace atmosphere during the annealing process. Traditionally, sand seals have been used to effect sealing of bell shaped furnaces. The sand seal (or conceptually, a water trough) is not acceptable as a seal between the base and the inner cover (The seal between the outer cover and the base is not critical and any conventional seal, including sand, can be used). Ceramic seals such as ceramic braid or ceramic blankets do not offer the consistent seal reliability necessary for use with pure hydrogen systems. By the process of elimination, this leaves elastomer seals as the type of seal which has the inherent sealing characteristics needed for the hydrogen application under discussion. Examples of prior art elastomer seal arrangements are set forth in Blackman U.S. Pat. No. 3,593,971; Freund U.S. Pat. No. 5,006,064 and Soliman U.S. Pat. No. 4,846,675. Though the seal arrangements discussed in the prior art patents will work, they will not consistently work over a long period of time and require frequent maintenance and replacement.
Separate and apart from any of the problems discussed above, cold rolled steel strip and sheet contain small amounts of rolling oils that cannot be completely removed with conventional equipment. Traces of rolling oil adhere to the steel surfaces in a tight and thin film. When the steel surfaces are then heated in the batch coil annealing process, the oil begins to evaporate and the oil vapors mixed with the recirculated heat transfer medium which, as discussed above, will consist of either an inert gas such as nitrogen, pure hydrogen, or any mixture of hydrogen and nitrogen. The oil vapor and the oil vapor mixture causes several secondary processes to take place with respect to the steel or base metal which are detrimental to the quality of the coil surfaces and have other operational disadvantages. The most detrimental effect is the deposition of carbon soot on the coil or sheet surfaces. This soot negatively affects paint adherence and surface cleanliness. Soot deposits are therefore closely controlled by the steel customer to assure a clean coil surface that can be readily painted after cleaning with a conventional phosphate wash. An example of such requirement is Ford Motor Company's engineering material specification ESB-M2P117-A entitled "Paint, Steel, Surface Cleanliness-Exterior" which limits soot deposits to less than 0.65 milligram per square foot.
After prolonged operation a typical coil annealing stand which, as noted above, includes an inner cover and a base becomes normally covered with significant amounts of soot and that soot is oily in areas of lower wall temperatures. Heretofore, several equipment suppliers have claimed that their equipment was capable of removing soot from coil surfaces. However, the experience of steel manufacturers has been that such surface cleanliness is difficult to replicate in a consistent manner. At present, the most reliable of all such techniques designed to eliminate or minimize soot or soot formation have proposed to rid the system of the soot or prevent the soot from forming in the first instance by soot oxidation. Suit oxidation is achieved by using small amounts of steam in a base of high hydrogen content atmospheric gas. The problem with this method (i.e., causing soot oxidation by steam) is the tight control of atmosphere composition which must be exerted to prevent surface oxidation of steel or, in the case of alloyed steels, to avoid intergranular oxidation of alloying elements. The oxidation method is, therefore, dependent on close control of the partial pressure of water vapor which must be measured and which can be determined by measuring the dew point of the recirculating atmosphere gas.
That is, soot formation and soot oxidation can take place only at rather high temperatures. The equilibrium value of the heterogeneous reaction of carbon with steam is close to unity at a temperature between 1220.degree. F. and 1270.degree. F. This means that a large excess of hydrogen at this temperature will also require a large amount of water vapor to form any appreciable amount of carbon monoxide. Removal of carbon monoxide at high temperatures and reduction of water vapor at lowered temperatures is very important or critical to complete removal of soot while preventing oxidation of steel surfaces. Very close control of surface temperature and gas composition is therefore required to obtain the desired, expected results. In fact, only a complicated or complex control system with feedback control of residual carbon monoxide concentration in the recirculating gases can assure clean steel surfaces.